The present invention relates to a cathode ray tube such as a color picture tube or the like.
A color cathode ray tube generally has a vacuum envelope comprising a glass-made face panel having a substantially rectangular display portion, a glass-made funnel joined to the face panel, and a cylindrical glass-made neck joined to the funnel. An electron gun which emits three electron beams is provided in the neck. A deflection yoke is mounted on the outside of the vacuum envelope so as to bridge from the outer circumference of the neck to the outer circumference of the funnel. The funnel has a small-diameter portion extending from the joint portion joined to one end of the deflection yoke, which is so-called a yoke mount portion.
On the inner surface of the face panel is formed a phosphor screen comprising dot-like or stripe-like phosphor layers which radiate in blue, green, and red. In the vacuum envelope, a shadow mask is provided to oppose the phosphor screen, and a number of electron beam passage apertures are formed in the shadow mask.
With the color cathode ray tube, electron beams emitted from the electron gun are deflected in the horizontal and vertical directions by horizontal and vertical deflection magnetic fields generated from the deflection yoke, and horizontally and vertically scan the phosphor screen through the shadow mask, thereby displaying a color image.
Color cathode ray tubes of a self-convergence inline type have been widely used as a kind of cathode ray tube as described above. In this kind of cathode ray tube, the electron gun is formed as an in-line type electron gun which emits three electron beams disposed on one same horizontal plane. Further, three in-line electron beams emitted from the electron gun are deflected by a horizontal deflection magnetic field of a pin-cushion type generated from the deflection yoke and a vertical deflection magnetic field of a barrel type, thereby to converge the three electron beams arranged to be in-line over the screen without requiring any special correction means.
In this cathode ray tube, since the deflection yoke is a source which consumes a large power, it is important to reduce the power consumption of the deflection yoke for the purpose of reducing the power consumption of the entire cathode ray tube. Specifically, to increase the screen luminance, the cathode voltage which finally accelerates the electron beams must be increased. In addition, the deflection frequency must be increased to respond to OA devices such as a HD (High Definition), a PC (Personal Computer), and the like, and leads to an increase of the deflection power.
Meanwhile, as for OA devices such as a PC and the like which are operated by an operator near a cathode ray tube, regulations concerning a leakage magnetic field which leaks from the deflection yoke to outside of the cathode ray tube have been strengthened. As a measure of reducing the magnetic field leaking from the deflection yoke, there has been a generally known method of adding a compensation coil. However, by thus adding a compensation coil, the power consumption of the PC is increased accordingly.
In general, to reduce the deflection power and the leakage magnetic field, the neck diameter of the cathode ray tube as well as the outer diameter of the yoke mount portion of the funnel to which a deflection yoke is mounted must be decreased so that the effective area of deflection magnetic fields is reduced and the deflection magnetic fields efficiently act on electron beams.
However, in a cathode ray tube, electron beams pass near the inner surface of the yoke mount portion of the funnel. Therefore, if the neck diameter and the outer diameter of the yoke mount portion are reduced much more, electron beams deflecting toward corner portions of the phosphor screen at a maximum deflection angle collide into the inner wall of the yoke mount portion, so that regions into which electron beams do not collide are generated on the phosphor screen. It is therefore difficult to reduce the deflection power by reducing the neck diameter or the outer diameter of the yoke mount portion much more.
If electron beams are kept colliding into a portion of the inner wall of the yoke mount portion, the temperature of this portion increases so that glass forming the funnel is melted, resulting in a risk of implosion of the vacuum envelope.
As a measure for solving the problems as described above, Japanese Patent Application KOKOKU Publication No. 48-34349 (corresponding to U.S. Pat. No. 3,731,129) discloses that the yoke mount portion of the funnel on which a deflection yoke is mounted is formed in a shape whose lateral cross-sections gradually change from a circular shape in the neck side to a substantially rectangular shape in the panel side, that is, formed in a pyramid-like shape. This structure is based on an idea that the electron beam passing area inside the yoke mount portion has a substantially rectangular shape when a rectangular raster is drawn on the phosphor screen.
If the yoke mount portion of the funnel is thus formed in a pyramid-like shape, the diameter of the deflection yoke attached to the outside of the mount portion can be reduced in directions of the long axis (or horizontal axis: axis H) and the short axis (or vertical axis: axis V). Therefore, horizontal and vertical deflection coils of the deflection yoke are arranged to be close to electron beams, and the electron beams can be efficiently deflected. As a result, the deflection power can be reduced.
However, as the lateral cross-section of the yoke mount portion of the funnel becomes rectangular to reduce efficiently the deflection power as described above, those portions of the yoke mount portion that are close to ends of the horizontal axis and to ends of the vertical axis become flat and may be easily deformed in the tube axis direction due to the load of the atmospheric pressure. Therefore, the strength of the vacuum envelope against the atmospheric pressure is lowered so that safety is lost.
Prevention of reflection of outer light on the surface of the face panel and easy view of images have been strongly demanded, and hence, flattening of the face panel has been required. However, since flattening of the face panel involves deterioration of the strength of the vacuum envelope, it is difficult to maintain strength sufficient for safety if the funnel having a yoke mount portion in a form of a pyramid-like shape is directly used as described above.
From the reasons as described above, there has conventionally been a problem that the yoke mount portion cannot be formed to be rectangular enough to reduce the deflection power sufficiently, or the yoke mount portion formed in a rectangular shape cannot be applied to a flat face panel. Therefore, with conventional techniques, it is difficult to manufacture a cathode ray tube which achieves both of sufficient strength against the atmospheric pressure and sufficient reduction of the deflection power.
As for techniques of forming the yoke mount portion into a pyramid-like shape, the present applicant produced two series, one of which provided a deflection angle of 110.degree. and a neck diameter of 36.5 mm in panel sizes of diagonal lengths of 18", 20", 22", and 26", and the other of which provided a deflection angle of 110.degree. and a neck diameter of 29.1 mm in panel sizes of diagonal lengths of 16" and 20", in 1970 or so. In those days, the outer surface of the panel is substantially spherical and the radius of curvature is as about 1.7 times large as the effective diameter of the screen, and the face panel is applied to a 1R tube. However, as for a cathode ray tube in which the outer surface of the panel has a radius of curvature which is as two or more times large as the effective diameter of the screen, its relationship with the shape of the yoke mount portion has not been apparent in relation to the bulb strength.